Capacitor discharge ignition circuit



Dec. 8, 1970 M- S. FISHER CAPACITOR DISCHARGE IGNITION CIRCUIT Filed Jan. 24, 1968 n 4 a: w a! r km a, Mfi J m United States Patent 3,546,528 CAPACITOR DISCHARGE IGNITION CIRCUIT Michael S. Fisher, Flemington, N.J., assignor to RCA Corporation, a corporation of Delaware Filed Jan. 24, 1968, Ser. No. 700,279 Int. Cl. H0511 41/36 U.S. Cl. 315-209 6 Claims ABSTRACT OF THE DISCLOSURE A capacitor discharge ignition circuit for an internal combustion engine in which the capacitor is charged to the same voltage between discharges thereof to produce ignition sparks of the same high intensity independently of the interval between the production of sparks up to a given engine speed.

BACKGROUND The sparks needed for ignition in an internal ignition are conventionally produced by separating a pair of 1gn1- tion points to break the primary winding circuit of an ignition transformer or coil in synchronism with the rotation of the crankshaft of the engine, whereby a properly timed ignition spark is produced by the secondary winding of the ignition coil. However, the conventional system has the disadvantage of requiring a capacitor across the ignition points and also has the disadvantage that the points may burn and need replacing after a few thousand miles of operation and further that the spark produced by such an ignition system may be weak when the battery is weak or during starting of the engine due to the low input voltage to the ignition system while the starting motor is drawing its rated relatively large current. Furthermore, the sparks may be weak at high engine speeds due to the lack of time, at such high speeds, between sparks for the current in the primar winding of the ignition coil to build up to its rated value.

Two types of circuits for transistorized ignition apparatus are known, both of which require an ignition coil. In one type, the energy necessary to produce the ignition sparks is stored in the ignition coil, whereby the coil must be designed for energy storage purposes and also the coil must be designed to act as a pulse transformer. The result is that the transistorized ignition using the coil as an energy storage device does not produce optimum results particularly at high speeds and that therefore it may be advisable to replace the standard ignition coil by a special coil designed for transistorized ignition of the coil energy storage type.

The second type of transistorized ignition for internal combustion engine includes a capacitor storage means. In this type of ignition, due to the fact that at different engine speeds there are different durations of time between ignition sparks, the energy storage capacitor which supplies energy for the ignition sparks may be charged to different voltages under different engine operation conditions, whereby the spark produced by the known capacitor discharge type of ignition circuit may not be uniform under all conditions of engine operation.

It is an object of this invention to provide a capacitor discharge ignition system in which the energy storage capacitor is charged to the same voltage between sparks up to a speed of operation of the engine so great that the capacitor does not become fully charged between sparks.

SUMMARY According to the invention, the variable automotive direct current supply is inverted and the resultant alternating current is transformed up to a higher alternating voltage. The high alternating voltage is rectified and is used to charge a storage capacitor. A discharge means is provided to discharge the capacitor through. the primary winding of an ignition coil in synchronism with the rotation of the engine. The voltage across the storage capacitor controls the inverter in such a manner as to cause charging of the storage capacitor to the same predetermined voltage regardless of the length of time between sparks up to a given engine speed and also to the same voltage regardless of the voltage of the automotive direct current supply. Furthermore, charging of the storage capacitor is prevented while the discharge means is conductive to permit the discharge means to become nonconductive so that the cycle may be repeated.

SHORT DESCRIPTION The invention may be better understood upon reading the following description in connection with the accompanying drawing, the single figure of which is a circuit diagram of a capacitive discharge ignition system embodying the invention.

DESCRIPTION The positive terminal 10 of the automotive direct current supply (not shown) is connected by way of an ignition switch 12, a fuse 14 and a starting resistor 16 all in series to the collector of an NPN transistor 34 whose emitter is connected to system ground 20. The collector of the transistor 34 is also connected to the cathode of a rectifying diode 22 whose anode is connected through a resistor 24 and a winding 26 of a three winding transformer 28 to ground 20. A resistor .30 is connected across the diode 22. The collector of the transistor 34 is also connected to the base of an NPN transistor 32. The base of the transistor 34 is connected to the anode of a voltage standard device such as a Zener diode 36, whose cathode is connected by way of a resistor 38 to ground 20. The junction of the base of the transistor 34 and the anode of the diode 36 is connected through a resistor 40 and a resistor 42 in series to the junction of the fuse 14 and the resistor 16. The junction of the resistors 40 and 42 is connected to one, 44, of a pair of ignition points 44 and 46, the other, 46, of which is connected to ground 20. The junction of the cathode of the diode 36 and the resistor 38 is connected through a resistor 48 to a plate of a storage capacitor 50.

The collector of the transistor 32 is connected via a resistor 9 to the junction of resistor 16 and fuse 14, and the emitter of the transistor 32 is connected to the base of an NPN transistor 52. The collector of the transistor 52 is connected to the junction of the resistors 16 and 42 by way of a second winding 54 of the three winding transformer 28. The collector of the transistor 52 is connected to ground 20 through a frequency determining capacitor 56. The emitter of the transistor 52 is connected to ground 20. A feedback resistor 8 is connected between the winding 26 and the base of transistor 52..

The third winding 58 of the transformer 28 is connected between ground 20 and the anode of a diode 60 whose cathode is connected to the anode of a capacitor discharge means which comprises a switching device such as a silicon controlled rectifier (SCR) 62. The cathode of the diode 60 is also connected to the junction of the resistor 48 and the capacitor 50.

The cathode of the SCR 62 is connected to ground 20 and the control electrode thereof is connected through a resistor 64 to the junction of the resistors 40- and 42. A capacitor 66 is connected between the control electrode and ground 20.

The cathods of the diode 60 is connected through the storage capacitor 50 and the primary winding 68 of the ignition coil 70 to ground 20. The secondary winding 72 of the coil 70 is connected between ground and the distributor (not shown) of the engine. A commutating diode 74 is connected across the primary winding 68, the cathode of the diode 74 being connected to ground.

As will be explained, the elements 8 to 74 comprise a capacitor discharge ignition system. Since the number of ignition sparks produced in the operation of an engine is a measure of the speed of rotation of the engine as distinct from the speed of the vehicle, a means generally indicated by the reference character 76 is provided to count the ignition sparks and therefore to indicate the number of revolutions per minute of the engine, that is, to act as a tachometer.

The tachometer 76 includes an NPN transistor 78 whose collector is connected through a load resistor 80 to the junction of the fuse 14 and the resistor 42 and to the cathode of a voltage standard device such as a Zener diode 82 and to one plate of a blocking capacitor 84. The base of the transistor 78 is connected through two resistors 86 and 88 in series to the ungrounded ignition point 44. The junction of the resistors 86 and 88 is connected through a small capacitor 90 to ground 20. The emitter of the transistor 78 is connected to ground 20.

The other plate of the capacitor 84 is connected to the anode of a diode 92 and to the cathode of a diode 94. The cathode of the diode 92 is connected to ground 20. The anode of the diode 94 is connected by way of milliammeter 96 to ground 20. A calibrating resistor 98 is connected across the ammeter 96.

The opeartion of the ignition circuit is as follows: When the engine is to be started, the ignition switch 12 is closed, applying operating potentials to the several elements disclosed. The starting motor (not shown) causes the engine to rotate and the points 44 and 46 to open and close in the usual manner in synchronism with the rotation of the engine.

While the points 44 and 46 are closed, the junction of the resistors and 42 is at ground potential, whereby the control electrode of the SCR 62 is at a low potential, and the SCR 62 cannot become conductive. The resistor-capacitor network 64, 66 acts to decrease the amplitude of pulses to the control electrode of the SCR 62, whereby any spike voltages that may be produced during the operation of the disclosed circuit does not damage the SCR 62. Also due to the action of the resistor 40, when the points 44 and 46 are closed, the transistor 34 responds to any control voltage that is applied to the base thereof as will be explained.

Feedback occurs from the collector of the transistor 52 to the base of the transistor 32 due to the coupling be I tween the windings 26 and 54. Due to the action of the starting resistor 16, starting current is fed to the base of the transistor 32, whereby, when the gain of the loop comprising the transistors 32 and 52 and the windings 26 and 54 is high enough, the oscillator comprising these elements will start oscillating. As the current in the transistors 32 and 52 increases, the voltage fed back to the base of the transistor 32 by the windings 54 and 26 is such as to render the transistors 32 and 52 highly conductive. The current through the transistors 32 and 52 increases to a saturation value and then levels olf. As soon as the current in the transistors 32 and 52 levels off and starts decreasing, reverse bias is applied to the base of the transistor 32 whereby the transistors 32 and 52 become suddenly nonconductive. A high ringing voltage is induced in the coil 58 which charges the capacitor by way of the rectifiers 60 and 74, the plate of the capacitor 50 that is connected to the diode 60 becoming positive. The frequency of the ringing is determined by the inductance of the coil 26 and the capacity of the capacitor 56. The resistor-diode network 22, 24 and 30 acts to limit the reverse bias applied to the base of the transistor 32 during feed back to a safe value. The use of the two transistors 32 and 52, due to their well known configuration, cuts down on the required base drive current for the two transistors 52 and 32 under the amount of base drive current that would be needed by one transistor 52 only.

A portion of the voltage across the capacitor 50 is applied, by action of the potentiometer comprising resistors 38 and 48 to the Zener diode 36. When the voltage ap plied to the Zener diode 36 is enough to break it down, the transistor 34 becomes conductive, shunting away feed back current from the base of the transistor 32. The frequency of the oscillations produced by the oscillator including the transistors 32 and 52 is increased, however the voltage amplitude thereof is reduced and the ringing voltage is reduced whereby he charging of the capacitor 50 is reduced. At stabilization, that is, when the voltage applied to the Zener diode 36 is just higher than its break down voltage, the energy fed to the capacitor 50 is just enough to supply leakage losses. Therefore, the capacitor 50 is charged to a predetermined voltage and is held at this voltage while the points 44 and 46 are closed.

The points 44 and 46 open. A positive voltage is applied to the base of the transistor 34 whereby it becomes fully conductive and shunts so much current way from the base of the transistor 32 as to prevent oscillations of the oscillator comprising the transistors 32 and 52. Therefore, while the points 44 and 46 are open, the oscillator comprising the transistors 32 and 52 is prevented from oscillating and charging of the capacitor 50 is prevented. At the moment that the points 44- and 46 are opened, a positive potential is applied to the control electrode of the SCR 62, rendering it conductive. Therefore, the charge of the capacitor 50 discharges through the SCR 62 and the primary winding 68 of the ignition coil 70 in series. This causes an ignition voltage to be induced in the secondary Winding 72. When the current in the primary winding 68 reaches its peak and starts to reduce, the voltage induced thereby in the primary winding 68 is shunted away from the capacitor 50- by action of the commutating diode 74, that is, the capacitor plate that is con nected to the diode 74 is clamped to ground by the diode 74. Since the capacitor 50 is discharged and is not being charged (the oscillator comprising the transistors 32 and 52 being disabled), no conduction sustaining current is applied to the SCR 62 and it becomes non-conductive. When the points 44 and 46 close again, the oscillator including the transistors 32 and 52 again starts oscillating to charge the capacitor 50 to a regulated voltage, and the cycle continues.

The operation of the tachometer 76 is as follows: Each time the points 44 and 46 are opened, a positive pulse is applied to the base of the transistor 78 by way of the resistors 86 and 88. The capacitor 90 is small such that it smooths out peak voltages that may be applied to the base of the transistor 78. The capacitor 90 should not be large enough to shunt away the impulses caused by the opening and closing of the points 44 and 46 from the base of the transistor '78. The Zener diode 82 acts as a voltage regulating means to prevent changes of calibration of the meter 96 with changes of voltage supply. Therefore, an oscillating voltage appears across the diodes 92 and 94 and the meter 96 whose amplitude is substantially constant and whose frequency depends on the number of sparks per second produced by the described ignition circuit. The ammeter 96 measures a current depending in amplitude on the frequency of these last-mentioned oscillations and, therefore, indicates the speed of rotation of the engine, thus, operating as a tachometer. Variation of the variable resistor 98 calibrates the reading of the meter 96.

Variations of the above described ignition circuit can be made without departing from the invention. For example, instead of a pair of points 44 and 46 that are opened and closed in synchronism with the rotation of the engine, other means may be provided for applying a positive pulse to the control electrode of the SCR 62, and for simultaneously applying an oscillator disabling voltage to the base of the transistor 34, at intervals in synchronism with the rotation of the engine. Therefore, the above description is to be taken as illustrative and not in a limiting sense.

What is claimed is:

1. An ignition system for an internal combustion engine, comprising:

an ignition coil having a winding;

an oscillator including an amplifier and means for feeding back the output of said amplifier to its input;

a rectifier coupled to the output of said oscillator;

a storage capacitor connected to be charged through said rectifier;

a switching device connected to discharge said capacitor through said ignition coil winding;

means to sense the voltage across said capacitor;

means responsive to said voltage sensing means to vary the output of said oscillator to vary the charging of said capacitor;

triggering means responsive to the ignition switching signals of said system to render said switching device conductive to cause discharge of said capacitor through said winding; and

means to prevent charging of said capacitor by preventing oscillation of said oscillator while said ignition signals are present.

2. An ignition system in accordance with claim 1 wherein said switching means is a silicon controlled rectifier having a pair of main electrodes and a control electrode, said main electrodes connected in series with said capacitor and said winding, and

means for causing application of a potential to said control electrode to cause it to become conductive in response to said ignition signals.

3. An ignition system in accordance with claim 1 or 2 wherein said means to prevent charging of said capacitor by said oscillator includes means to reduce the feedback of said oscillator responsive to said ignition signals.

4. In an engine ignition system, comprising a source of ignition switching signals; an ignition coil having a Winding; a storage capacitor and a switching device connected in series with each other and with said winding; adjustable means for charging the capacitor, said capacitor charging means including an oscillator and a rectifier coupled to the output of said oscillator, said storage capacitor being connected to be charged through said rectifier; means for sensing the voltage across the capacitor; means responsive to said voltage sensing means to vary the output of said oscillator to vary the charging of said capacitor to maintain the voltage across the capacitor at a predetermined value while the switching device is nonconducting; and means responsive to said ignition signals to render said switching device periodically conductive to cause discharge of said capacitor through said winding, the improvement comprising:

means to prevent charging of said capacitor by preventing oscillation of said oscillator while said ignition signal is present.

5. An ignition system in accordance with claim 4, wherein said switching device is a silicon controlled rectifier having a pair of main electrodes and a control electrode, said main electrodes connected in series'with said capacitor and said winding, and

means for causing application of a potential to said control electrode to cause it to become conductive in response to said ignition signals.

6. An ignition system in accordance with claim 4 or 5, wherein said means to prevent charging of said capacitor by said oscillator includes means to reduce the feedback of said oscillator responsive to said ignition signals.

References Cited UNITED STATES PATENTS 3,248,605 4/1966 Tomkinson 315-241 3,263,124 7/1966 Stuerrner 315-209 3,329,867 7/1967 Stearns 315-209 3,318,295 5/1967 Byles 315-209 3,375,403 3/1968 Flieder 315-209 OTHER REFERENCES Electronics, Two Added Transistors Reduce Ignition- System Current Drain, by Scheel, May 1967, p. 87.

JERRY D. CRAIG, Primary Examiner US. Cl. X.R. 123148 

